I, in general would avoid it. There are times it can come in handy, but usually I would suggest having your lord hide inside a unit so that it can't be singled out by weapons that instant kill him. Then make him cheap and fast as you suggest. There are times where I do see a benifit of it: expecially when you know you've got to kill something that might instant kill you first, but these would be exceptions that you wouldn't be able to account for in a tournement style list.

So that would be one vote for cheap and fast. . . heck most of the time my army is lead by a lieutenent. (of course I play LatD so that would be Arch Heretic but still. . .)

Unless you go for the cheap lieutenant option, most lords come in in the 180-210 points range once fully tooled out. Daemonic rune sinks even more points into an already expensive model.

Daemonic Gifts are very pricey for a lord and 100 points are easily spent. most people don't go for the daemonic rune because it only helps you against an event that might not happen. At the points cost of the rune that is lots of points wasted. If your lord does not have daemonic stauture you can "hide" him in a squad (as previously mentioned by JORMAGI) and spent the points on something he might actually use (D. Strength or Mutation or some Wargear or magical stuff or whatever else comes to mind.).

i avoid the rune as i prefer a speedy devastating CC lord rather than a foot slogger that can take punishment but have compromised ability, my lord still comes out at 196pts when in large games but i can get more for my points not taking the rune

I'd say it depends.
Since the hidden powerfist is not that an uncommon tactic, and bearing in mind insta-kill-weaponry is in every mans reach... Sometimes it's worthwile.

Will your Lord be moving around on his own? Is he vital for your tactics? Is he already very expensive? If you go "Indeed, that's my Lord" then it might be worth those extra points.
I've used it from time to time with great success, on other ocasions completely in vain.

Try it for a couple of games so you can see how it suits your way of play.

All I'll say is: the Lord I'm building at the moment - has it. (it's a biker Lord for my TS)